Keep in mind that propellers only provide thrust, not lift. Airfoils are what provide lift, and the amount of lift they provide is dependant on how much air flows around them. How much air flows around an airfoil is dependant on two things: the density of the air, and the speed of the craft. In thinner air, the only way to increase your lift is by increasing your speed. At 328,000', there is a smidgeon of atmosphere left, but you'd have to be going insanely fast to generate any lift with it. Considerably faster than orbital velocity, in fact. And there are some darned good reasons why you've never seen propellers accelerate anything to even mach 1, let alone mach 40...

That's true, but with all things being equal, that is if the engine and prop are designed to be as efficient at altitude as at sea level, the same amount of thrust provided would make it go faster. By going faster because of less drag, the same amount of lift will be obtained at altitude as at sea level. It's true that the speed would be much greater. In fact, at 100,000' if the airspeed indicator was indicating 50 MPH, you would be going almost 454 MPH. The airplane wouldn't know it. It would perform just as though it were at sea level.

It's true that the speed would be much greater. In fact, at 100,000' if the airspeed indicator was indicating 50 MPH, you would be going almost 454 MPH. The airplane wouldn't know it. It would perform just as though it were at sea level.

It wouldn't melt. The heat generated by friction moving through the air is proportional to speed and the density of the air. The speed necessary to maintain lift at altitude wouldn't develop any more heat due to friction than it would at a low altitude while flying slow. If the airspeed indicator indicated say 50 MPH at low altitude the heat generated by friction would be exactly the same as it would be at a much higher altitude if the airspeed indicator was indicating 50 MPH, even though it was going many times faster.

By going faster because of less drag, the same amount of lift will be obtained at altitude as at sea level. It's true that the speed would be much greater. In fact, at 100,000' if the airspeed indicator was indicating 50 MPH, you would be going almost 454 MPH. The airplane wouldn't know it. It would perform just as though it were at sea level.

The airplane might not know the difference, but the propeller and the engine sure would. If the propeller has to do, say, 600 RPM at sea level in order to give you an airspeed of 50mph, then at 100,000' it would have to do 5,450 RPM; at 200,000', where an actual speed of 3,480 MPH is required to maintain the same dynamic pressure, the propeller would have to do 41,760 RPM; at 300,000'... well, my atmospheric calculator only goes up to 230,000', but the atmosphere is thinning fast, and already you'd need 7,770 MPH and 92,400 RPM...

This is assuming that air pressure behaves the same at all velocities, which it actually doesn't. At supersonic speeds, it behaves somewhat differently, and at hypersonic speeds (which is what we're talking about), it behaves radically differently. Even if the density is very low, it will form a plasma around the vehicle (actually, around the propellers long before the vehicle, since they must move so much faster.) I honestly don't know much about the aerodynamics of plasmas, except that they are as different from gas as gas is from water. I believe that they actually reduce drag considerably (which is a plus for rockets), and correspondingly reduce lift (a minus for propellers and airfoils).

This is also ignoring the fact that a, say, 6'-radius propeller, rotating at 92,400 RPM, would be experiencing approximately 1,500,000 G's of centrifugal force. Try engineering for that!

You are right! I don't know what a prop could be made out of to withstand the G's! Sure wouldn't want to be around when it was run up to speed!

Above 90 Kilometers, the speed of sound is a non event.

NASA really doesn't know much about aerodynamics above 100,000'. That's why they are looking for a vehicle to fly between 100-120,000' which is about the same atmospheric density as Mars.

The military did research on supersonic propellers several years ago. It was found that a supersonic prop can be designed that is pretty efficient. Of course you know who won out....... The jet engines.

Considering this.............. I would think that a large diamater (30' or more) prop with adjustable RPM and PITCH would work as the rpm would be a lot lower and the G's would be within reason. If a supersonic prop would be effective above 90 kilometers altitude then it could be shorter in length. The chord would need to be large too.

The reason for this thinking is that the SR-71 could only reach 85,069' in level flight. It's true, that was a long time ago but I feel it was redesigning the jet engine about as far as it could go in the sense of the adjustable nose to get the shock wave located properly for maximum thrust. The jet engine would continue producing thrust if it had something to compress. If I remember right, someone said at 85,069' the engines were producing about the same thrust as they did sitting on the ramp at idle.

My question is....... Would you rather have the job of providing enough air at altitude for a jet engine or an internal combustion engine that produced the same equivelent horespower? If it is a turboshaft engine and a regular internal combustion piston engine (apples to apples) the turboshaft engine consumes many times more air than the internal combustion engine. Since it takes several stages of compresson to get atmospheric air for the engines, I think the IC engine wins.

why not use a propeller and a rocket Posted on: Thu Aug 28, 2003 9:00 pm

I guess you could use a propeller to get you to altitude and then use a rocket.

Below is a cool plane.
[url]http://www.cias.org/Sections/AircraftDetail.asp?ID=23[/url]

Imagine a bigger version of this. The turboprop replaced by a rocket turbine powered propeller, similar to the turbine drives found in torpedos, i.e. ones, which use liquid oxidisers instead of air and so have no heavy turbo compressors, just drive turbines. The engine mass will be much less at burnout (less dead weight when switching to rocket power) than if a conventional turboprop was used. Launch it vertically to reduce wing size, which only has to be big enough to support the empty landing weight.

Maybe a ducted fan could replace the propeller to increase the final velocity of the non rocket powered phase of the flight.

The turboprop replaced by a rocket turbine powered propeller, similar to the turbine drives found in torpedos, i.e. ones, which use liquid oxidisers instead of air and so have no heavy turbo compressors, just drive turbines. The engine mass will be much less at burnout (less dead weight when switching to rocket power) than if a conventional turboprop was used. Launch it vertically to reduce wing size, which only has to be big enough to support the empty landing weight.

Maybe a ducted fan could replace the propeller to increase the final velocity of the non rocket powered phase of the flight.

Kermit

I'm refraining my opinion from the rest of the thread, but this point is interesting. I think a prop would be unpractical though, because the rocket is quickly out of regime for efficient prop use, but the idea of a fan is much more interesting. Let's take the idea a bit further.

The best Isp achieved by modern rocket engines seem to be around 450, which is FAR more than any homebuilder of today can dream of. However, compared to a modern turbofan with the Isp of around 11.000 - 12.000, the <500 figure is pitiable.

Obviously, if rocket engine could be made to utilize the ambient amtosphere like a jet engine, the Isp for initial flight could be tremendously improved.

Here's my theory:

The heart of the engine is an annular-chamber aerospike engine, which already has a rather good Isp. The chamber is located about at the afterburner of a modified low-bypass turbofan or turbojet. A part of the chamber pressure is vented into a two-stage axial turbines that are housed inside the chamber torus. The turbines act as in regular jet engine, providing shaft power for compressors and auxiliary equipment. The original combustor could be utilized to give an extra boost to the engine. Also, the (re)heating the air in the afterburner chamber by the rocket engine exhaust would improve the efficiency of the engine as the surplus heat would be used to expand the passing air.

Obviously, this kind of solution would be out of reach for homebuilders of today too. However, with the efforts by NASA, Williams Int. and P&WC to bring jet engines into General Aviation, it might not be too far fetched to imagine such a conversion to emerge. After all, there's more demand for General aviation jet engines than General astronautix Space Shuttle Main Engines...

Nope but we do plan to win the N-Prize with one. Our Quadcopter design is for an altitude or 100kft and is designed after the Helios aircraft propellers. We have a prototype under construction right now. We do have the first prototype propeller on display on the Team Prometheus Facebook page. The will be photo's soon of the prototype platform (next few days) and it is remote/autonomous. Testing will begin soon!

We've had our propellers up just over 80,000 feet. The performance was outstanding. This is a two meter prop designed specifically for 100,000 feet.This spring we'll have a pair being tested at 110,000 feet. We have a new propeller in the works that's designed for 140,000 feet. This is the highest I can figure out how to make it work.

To make a propeller worker much higher than 140,000 feet you would need to actively control the boundary layer just for a start. Not many molecules up there and they all behave as Newton intended rather than Bernoulli.

Wow! JP I like the way you think! I am using an Arducopter as the basis for our platform. Ours uses 4 4' rotors for flight at 100kft. I'm using a vacuum chamber for testing flight. We plan to fly it as is at low altitude for a while to learn how to fly it first and for testing the launch. Our first Rockoon is nearly ready! NSE 5.1 will be that launch and NSE 5.2 We will fly the Quadcopter to 100kft NSE 6 we will use them both together. 5.2 will also test the FPV/OSD and ground station for live video and telemetry using a 20 watt 900 Mhz transmitter and auto tracking using the 3 meter dish we designed a while back with the standard Ardustation ground station at two Hz and a wheel antenna on the payload. Our LNB is a helical pick-up to match polarization to the wheel. Our backup telemetry is going to be the 2-meter APRS unit we tested with NSE 5 The remote control was built by Thomas Scherer of Copenhagen Suborbitals the Rockoon is using a Beeline 2-meter APRS tracker we are testing on NSE 5.1 and a copula HD cameras. Jerry Irvine of U.S. Rockets built the rockoons and Adrian at Featherweight altimeters modified flight controler. Sparkfun and DIY Drones as well as Total Impulse Rocketry, U.S. Rockets, Big Red Bee, Featherweight Altimeters. Scherer UHF and True R/C are now Sponsoring Team Prometheus we are now using OTS components. If this all works out anyone that wants to and can afford it will be able to make a space shot with OTS components and recover there data and video! 3 long years of work for me and a lot of help to finally make a space shot on a shoestring. Then hopefully we can somehow get the funding to build the N-prize size platform. I have the first stage "P" booster and the $16,000 zero pressure balloon that can lift it. I'm so damn close now I can taste it! I cant believe I've made it this far on sheer will and determination. If I had one good backer for just a little more funding we could punch a hole in the sky! Not only win the N-Prize but the NASA Centennial Challenge on the second shot! Within a year we can do it. If we get some funding. If not I'm still going to do my best you never know maybe our team will pull it off anyway! GO! TEAM PROMETHEUS! GO! "Today's the Day" The sky is not the limit of our dreams! Only you limit your dreams. Come on people we can do this! It's not that tough! Do the impossible thing! Then do the unthinkable with a grin!

Hahahahaha JP you rock! every time I get some new Idea and start building it you've already done it! lol the cloud chamber thing was the real shocker hahahahaha oh man! I hope we get to sit down together some day and chat for a while. All I can say is YOU ARE ON THE RIGHT TRACK! You and I are in total agreement on many aspects of how to get into space cheaper. YOU ARE RIGHT! That's how we are going to do it. Not the throw money away method. You'll see one day I hope before we get to old, your dreams will become reality. Everyday space travel cant use big rockets we need a lot of smaller "cars and trucks" to get into space for real. Right now I think people laugh at what we are doing but you'll see I hope before we die they'll finally figure out what we already know. All I can say is Kick as much ass as you can JP. See ya on the other side of the atmosphere. Soon I hope : ) "Today's the Day!" Go! JP Aerospace Go!

Monroe

Nice prop JP USE IT!

Heed this you stupid investors out there your WASTING your money dumb ass conventional launch services are way to dirty and expensive we are going to have to use High altitudes and Ion or Plasma engines. For the cars and trucks of the future. You listening kiddo's study your physics and your aerospace work together. Your funding the wrong people again! Cant you see how stupid wasting all that fuel is? Dumb! Not going to happen don't fund that crap for another 50 years! It's futile cant you see that? Not going to happen! Wake up! That's an OLD WAY that's Dinospace! WELCOME TO NEWSPACE! brought to you from a nearspace way of life! WHY WASTE MONEY ON SUBORBITAL ROCKETS WHEN NEARSPACE IS RIGHT AT YOUR FINGERS! FOR NEXT TO NOTHING IN COMPARISON! AT NEARLY 1/1000th THE COST? Work on ways to Capitalize on the much easier way! NOT THE THROW MONEY AWAY WAY! The only reason we went that way in the first place was it was the quick and dirty way! It was a RACE! This is an endurance thing now not a race! A durable long term steady supply of high altitude vehicles going onto space. Not one shot wonders that burn so much fuel we'll kill off our world polluting the sky's now. Let's just skip that part and get to the good stuff what do ya say? Can we do that now?

I don't totally agree with JP or I'd just go join his team but he is on the right track if your not going to fund me at least look at JP Aerospace as an alternative? I mean this HOW STUPID DO YOU HAVE TO BE? DINOSPACE IS DIEING! It cant do what we are going to do IT COST TOO MUCH ___ ))) __ MONEY! It's obvious! DUH! Have a little vision people! It cant work that way much longer. It uses too much burning fuel we have to stop burning fuel it's too dirty! And wasteful and dangerous to boot! It's insanity!

We have other ways your just bent on throwing your money away arent you? Just want to see a big dangerous rocket fly? It's not practical it's blatantly obvious with all the exhaust from cars and jets now you want to dump tons of rocket fuel into the air we breath too?

Yeah that's going to work. That's the direction we are going to fund. That's the right thing to do. NOT!

Pull your head out! We are on the right path here. Like it or not fellas JP and I will end up being on the right path you'll see. Your so smart though so smart. Scary. I don't clam to be some super smart guy just practical. Don't like our method or think it's practical? Better get used to it and find a way to make it practical because it's going to be the only way to do it. Mark my words it's going to be the way we do it. You'll see. You need a TV in there where your head is DINOSPACE cant work. No way.